P13 Braking and Momentum Flashcards
stopping distance
the distance a vehicle travels to safely come to a stop after the driver has spotted a hazard
= thinking distance + braking distance
30&50&70 mph stopping distance
30) 9 + 14 =23m
50) 15 + 38=53m
70) 21 + 75=96m
compare the effect of speed on thinking and on braking distance
thinking prop to speed
braking prop to speed squared
factors affecting braking distance
speed of vehicle, road conditions, conditions of brakes and tyres
eg: less friction between tyres and road - skidding - increases braking distance
calculate energy transfer when braking
braking force (N) x distance (m) = energy transfer (J)
factors affecting reaction time
tiredness, drugs, alcohol, distractions
ways to measure reaction time (2 ways)
computer - time how long someone takes to respond to a sound/image on the screen
ruler drop test - ruler is dropped between someone’s fingers and the distance it falls before they catch it is used to calculate their reaction time
the faster a vehicle moves / the greater its mass: (4 things)
KE greater
more work done to transfer energy to slow it down
greater braking force required to stop it in a certain distance
the greater distance required to stop it with a certain braking force
calculate deceleration
same way as acceleration!!
(braking) F = ma
- if braking force is very large, the brakes may overheat or the car may skid - driver will lose control
force in a car crash is…
change in momentum / time
(because F = ma and a = change in v / t)
vehicle safety features and how do they work
cars: air bags, seat belts, crumple zones
+ cycling helmets and gymnastics crash mats
f = rate of change in momentum
so:
if time increases - the smaller the rate of change in momentum - the smaller the force
experiment to test effect of road conditions on stopping distance
raise one end of the ramp by a height h
cover the floor at the other end of the ramp with a type of surface
place a trolley at the top of a ramp and releases it
measure the distance the trolley travels from the bottom of the ramp to the
place that it stops
repeat the experiment twice more with the same surface
replace the surface with a different material and repeat the experiment with
ramp at same height h
identify outliers; do not include them in the calculation of mean
exam q!
skater A is stationary, skater B is moving towards skater A. when they collide they link arms and move off together with stopping. the velocity is different. why? (4)
momentum is conserved
the mass of the two skaters together after the collision is bigger than the mass of
the single skater before the collision
so the velocity of the two people together will be less than 5.0 m/s
assuming the system is closed/there are no external forces acting
